Orthopedic implants, such as knee implants, typically include a tibial implant and a femoral implant. The tibial implant generally includes a load bearing component (such as a tibial plate) which may be connected to a stem to be received in the tibial canal to stabilize the load bearing component. The femoral implant includes a load bearing component (such as a condylar component) which is connected to the distal end of the femur. The femoral load bearing component is sometimes connected to a stem which is received in the femoral canal to stabilize the load bearing component.
The tibial plateau and the condyles of the femur bearing on the tibial plateau act similar to a hinge within the knee to allow bending and other movement of the knee. Optimal movement and operation of the knee is achieved when both the tibial plateau and the condyles are aligned with the mechanical axis of the leg (defined as the line from the center of the femoral head to the center of the ankle). The tibial load bearing component and the femoral load bearing component ultimately cooperate with each other to replicate as closely as possible the action and relationship of the tibial plateau and the condyles of the femur bearing on it. Just as with an actual knee, the success of such implants depends at least partially on their positioning within the knee so that the longitudinal axes of the tibial load bearing component and the femoral load bearing component are aligned with the mechanical axis of the leg.
Very briefly, to implant the tibial and femoral implants into the knee, the surgeon first reams the intramedullary canals of the femur and tibia. Then, the proximal surfaces of the tibia and the distal surfaces of the femur are prepared for receiving the implants. Trial reduction usually follows to assess bone preparation and to select properly sized and configured tibial and femoral implants. The actual tibial and femoral implants are then assembled and implanted into the knee.
The tibial and femoral canals are not always aligned with, but rather are generally offset parallel a distance from, the mechanical axis of the leg. Because of such parallel offset, when a stem is inserted into an offset canal, the attached load bearing component is not aligned with, but rather is offset from, the mechanical axis of the leg. To accommodate for such offset canals, some stems, such as those disclosed in U.S. Pat. No. 5,290,313 to Heldreth and PCT Application No. WO 00/06056, are formed with two longitudinal axes offset from each other so that one axis runs through the center of the tibial or femoral load bearing component and the other axis is aligned with the offset tibial or femoral canal. In this way, when a stem is inserted into a canal offset from the mechanical axis of the leg, the longitudinal axis of the load bearing component may still be aligned with the mechanical axis of the leg. Although offset, the axes of such stems, however, are still parallel to each other.
While the stem may connect directly to the tibial or femoral load bearing component, intermediate stem extensions, such as those disclosed in U.S. Pat. No. 5,782,920 to Colleran, have been used to connect the stems and components. Colleran discloses a tibial prosthesis that includes a tibial tray, a stem, and a stem extension that connects the stem to the tibial tray. In contrast to the Heldreth-type stems with offset longitudinal axes, the stem in Colleran has, like most stems, a single longitudinal axis. The Colleran stem extension, rather than the actual stem, is the component which features offset longitudinal axes. When the system is assembled, the desired parallel offset between the stem and the tibial tray is obtained by virtue of the stem extension. Again, however, the axis of the stem is parallel to the axis of the tibial tray and to the mechanical axis of the leg.
Studies have shown, however, that in addition to being offset from the mechanical axis, the tibial and femoral canals are not always disposed parallel to the mechanical axis of the leg. Rather, across a population of humans, a valgus bowing of the tibia exists from about 1.63°+/−1.57° relative to the mechanical axis. Consequently, if a stem oriented parallel to the mechanical axis is inserted into the bowed tibial canal, the stem can impinge on the lateral cortex of the tibial canal proximal to the knee and the medial cortex distal to the knee. Similarly, the femoral canal can bow posteriorly relative to the mechanical axis, which results in impingement by the stem of the anterior cortex of the femoral canal in the diaphysis of the femur and the posterior cortex slightly superior to the knee. Such impingement can prevent adequate penetration of the canal by the stem and result in improper positioning of the tibial and femoral components in the knee.
Improper positioning of the component with respect to the bone can have grave effects, including stress shielding and bone loss due to nonuniform transfer of load from the bone to the stem, and can also limit range of motion. Insertion of a stem into an angled tibial canal may result in the misalignment of the tibial component with the tibial plateau so that a part of the tibial component hangs over the tibial plateau. Such overhang can lead to the tibial component rubbing the soft tissue surrounding the knee, causing irritation and pain. Moreover, a consequence of overhang by one side of the tibial component is underhang by the other side of the tibial component, so that the underhang portion of the component is resting on the softer cancellous bone instead of the harder cortical bone along the peripheral rim of the tibial plateau. The component consequently may sink into the softer bone, causing the entire component to tilt toward the side of underhang. This can jeopardize the stability of the implant.
Furthermore, the orientation of the femoral canal is such that when the stem and connected femoral cutting block is inserted, the femoral resection may notch the anterior cortex, predisposing the femur to fracture. More often, however, the orientation of the canal forces the femoral component's anterior flange to sit proud on the anterior cortex, thereby creating a gap between the anterior flange of the component and the anterior cortex of the femur. Traditionally the gap has been filled with bone cement, bone graft, or a metal shim or augment. This step of filling the gap adds time to the procedure.
To prevent such adverse effects by better positioning the stem into the bowed canal, surgeons often will select a smaller diameter stem that can be inserted the requisite distance into the canal without impinging the bone unduly or in undesired places. However, use of a smaller diameter stem compromises the fit between the stem and the canal, which can lead to movement of the stem within the canal. Such movement can result in undesired shifting of the attached load bearing component relative to the bone so that the component is located in an undesirable position within the knee. It can also result in instability of the prosthesis in general, excess wear, and other adverse effects.
Surgeons who choose not to downsize the diameter of the stem sometimes, perhaps unknowingly, rotate the femoral or tibial load bearing component and the attached stem to orient the stem relative to the canal to permit deeper penetration of the canal by the stem. This can also result in undesirable positioning of the load bearing component and consequent effects such as those disclosed above.